I'll play, the question seems legit, and I'd be interested in the answers of others.
The most sketchy run I did was on a run I've done a thousand times. I would have claimed I know every inch of it and clould ride it blindfolded backwards, heavily drunk. Then there was this day with the thickest fog I've been in ever. You couldn't see the tip of your board. Getting off the chairlift, I was lost after few meters. I didn't know if I was riding up- or downhill, didn't expect bends to come and whats worst, one had no clue when to increase speed to make it over flats. We almost crawled down the run. What a relief when we finally managed to reach tree line and had a little lead, where we were